1. Field of the Invention
The present invention relates to a knocking sensor and, more particularly, to a non-resonance type knocking sensor for detecting a knocking of an internal combustion engine.
2. Description of the Related Art
The following structures are known as example of such a knocking sensor (as will be shortly referred to as the “sensor”). In one structure (of JP-A-2002-257624): an insulating member, an electrode plate, an annular piezoelectric element, an electrode plate and an insulating member are arranged sequentially in the recited order on a supporting flange, which is formed at the outer circumference of the lower end of a cylindrical portion of a cylindrical body; a ring-shaped weight (or a weight) is arranged on the upper insulating member; and the piezoelectric element is clamped on the supporting flange. In this structure, a vibrating load according to a knocking vibration is applied to the piezoelectric element by the weight so that the piezoelectric element may generate a voltage signal according to the knocking vibration. The signal voltage is extracted from connection terminals wired to and connected with the electrode plates on the two faces of the piezoelectric element thereby to detecting a knocking so that an ignition timing or the like is controlled to suppress the occurrence of the knocking.
In this knocking sensor, the means widely adopted for clamping to fix the piezoelectric element together with the individual members is a fixing structure using a nut, in which a washer (or a disc spring) having spring properties is arranged on a weight so that the piezoelectric element is fastened by screwing a screw member such as the nut into threaded portion formed in the outer circumference of the cylindrical portion. There is also a knocking sensor, which has fixing means other than that screw type.
Specifically, the knocking sensor (of JP-A-11-173907) is provided with: a case worked from a metal plate to have a vertical inner circumference wall, a horizontal bottom wall and a vertical outer circumference wall concentric to the inner circumference wall to define an annular housing portion having an open upper end; an annular piezoelectric element housed on the bottom portion of the housing portion; an annular weight arranged in the housing portion on the upper side of the piezoelectric element and fitted and fixed on the inner circumference wall of the case by pressing it into the case; and a resin filler filling the cavity of the housing portion. This knocking sensor is made to have the structure, in which the annular weight is pressed and fixed in and a long the inner wall of the housing portion of the case for housing the piezoelectric element and so on so that the piezoelectric element is held by that weight.
In the technique disclosed in JP-A-2002-257624, the means for pressing and fixing the piezoelectric element is exemplified by screwing the nut into the threaded portion formed in the outer circumference of the cylindrical portion. Therefore, it is possible to acquire the reliability in clamping (or pressing) and fixing the piezoelectric element. However, this structure may cause a situation, in which fine metal chips (e.g., fine strips or powder) may be generated between the nut and the threaded portion in the outer circumference of the cylindrical portion by the friction between them. The fine metal chips may stick to the electrodes of the piezoelectric element or the electrode plates in the case thereby to cause a short-circuiting drawback and to invite failures.
In the press-in structure of the weight described in JP-A-11-173907, on the other hand, the weight is pressed in along the inner circumference wall of the case, in which the piezoelectric element is housed and arranged. This structure increases the probability of such a danger more seriously than the aforementioned nut screwing case that the fine metal chips (i.e., the metallic fine strips or powder making the case or weight) are formed by the friction (or wear) action or the scraping action between the weight and the inner circumference wall at the press-in time. Specifically, those metal chips highly probably intrude into the case in the press-in procedure. The chips stick to the electrodes of the piezoelectric element or the electrode plates in the case thereby to cause a serious problem that the short-circuiting drawback is invited to raise the probability of danger of failures.